As we were driving on the interstate we heard a very loud explosion type sound. it was night but in a well lit section of interstate. we weren't near any overpasses and were in the right lane of a three lane highway. we pulled over as soon as it was safe to see what had happened. when we checked the car we discovered our sunroof was shattered significantly. neither of us in the front seat saw anything coming towards us and there was no evidence that anything hit us. we were traveling about 75 miles an hour with cruise control set. the sun shade was closed at the time the glass shattered so none of the glass entered the vehicle.